In Vienna, Iran, the U.S. and other world powers are starting to negotiate over how to limit Iran's nuclear program, or as Iran sees it, how to lift the economic sanctions crippling the country. The goal is to have a comprehensive agreement this summer. That's when a temporary deal expires. The gaps are wide, the issues are complicated and it's believed this round of talks may be cut short. NPR's Peter Kenyon is in Vienna for the talks. And, Peter, with all this work to do, why does it look like the talks may end early?

PETER KENYON, BYLINE: Well, Robert, as important as these talks are in the longer term and in the bigger picture, this week, the widening bloodshed in Ukraine is demanding the attention of the EU foreign policy chief, Catherine Ashton. She's the one leading the talks here in behalf of the U.S., U.K., France, Germany, Russia and China. But she needs to be in Brussels for an emergency meeting tomorrow afternoon, according to Western diplomats. So at the moment, we're expecting these talks to wrap up in the morning sometime.

SIEGEL: Well, the nuclear talks began Tuesday. What do you know about how things are going?

KENYON: There were rumors of bumps in the beginning. Yesterday, we were hearing that things hadn't gone especially well at the opening session. But given how far apart the sides are, that wasn't a huge surprise. And since then, officials from both Iran and the West have described the talks as moving ahead pretty well roughly as expected. No one expected breakthroughs in this first round. And certainly, the opening positions are very far apart. So it indicates a long road ahead.

SIEGEL: Peter, remind us again of what these talks are supposed to accomplish and why leaders in Tehran and Washington aren't voicing much optimism about them accomplishing it.

KENYON: Well, this comes out of the preliminary agreement from last November, which enacted basically a pause in a lot of Iran's nuclear program. And this time, the talks are intended to put comprehensive limits to finish parts of the program in ways that assure the world of what Iran has always said, that it's not seeking a nuclear weapon.

To the West, that means limiting the number of centrifuges, the things that spin the uranium to enrich it, whether Iran can upgrade to more advanced centrifuges, which it wants to do, whether Iran really needs a heavy water reactor at Arak, which could produce weapons-grade plutonium if Iran chose to do that. And Iran's also supposed to address U.N. Security Council resolutions and things like possible past military dimensions to their program. It's a lot to ask in six months, possibly 12. It could be extended.

And you're right, the pessimism is showing strongly right now. Iran's supreme leader says these talks, he thinks, are going nowhere. President Obama gives them no more than a 50-50 chance.

SIEGEL: On the other hand, Iran's supreme leader has defended his negotiators from criticism. And President Obama has held off an effort in Congress to impose new sanctions against Iran. So there's obviously some political will to reach a deal. Do you think it's enough?

KENYON: Well, that is the huge question. It's definitely a high-risk, high-reward diplomatic foray. I mean, if you look at what happens with success, that really would mean Iran's re-entry into the global economy. That's a huge victory for Hassan Rouhani, the pragmatic Iranian president. It could insulate him somewhat from the hardliners in his country and maybe even lead to progress with the West on other tough issues.

For President Obama, resolving the Iranian nuclear dispute would be - have to be part of any discussion of his top foreign policy accomplishments. It would mean a big boost for those who favor slow and messy diplomacy over dramatic military interventions. So the odds may be very, very long, but the reward is potentially great.